SAN FRANCISCO (Reuters) - San Francisco's Center for Sex and Culture played host on Saturday to the city's annual "Masturbate-a-thon," an event its organizers said could draw up to 120 people from across the United States aiming to have a good time with themselves.

The event was organized to help raise funds for the center, and, according to its organizer, provide an outlet for safe sex for those who enjoy pleasuring themselves in a semi-public setting.

Carol Queen, director of the center, acknowledged that the event is unusual -- even by San Francisco's standards. The permissive city, which helped ignite a debate on gay marriage last year, tolerates many sorts of sexual behavior but masturbation seems a topic that is off-limits, she said.

"Even people who are sexually frisky ... might have the bias that many Americans do, that it's second-best sex, that it's something you do if you can't figure something else out," Queen said.

The Saturday night event also had a competitive side.

One New York man arrived shortly after 5 p.m. seeking to break the endurance six-and-a-half hour record set at last year's event. The rules allow for a five-minute break every hour.

The female marathon winner last year, Norine Dworkin, chronicled her experiences in the women's magazine Marie Claire, saying hours later the activity was "about as pleasurable as rubbing an elbow."

This year, others like Tony, who gave only his first name, attended the event to indulge in exhibitionist behavior in front of other people.

"I grew up believing that this is a horrible, nasty thing you shouldn't overdo," said Tony who is from California's Central Valley. "The whole point is coming out and making love to your best friend."

Melissa Gira, a former peep show performer who oversaw the evening's Web cast, was considering joining in, as she had done last year. "It's interesting to be sexual in public," she said. "These aren't things we're sexually socialized to do."